This invention relates to a method for converting undesirable environmental contaminants into environmentally acceptable materials. More particularly, the present invention relates to a biological method for converting organic compounds that are water and soil contaminants into innocuous compounds.
The field of the present invention will be described initially in connection with the contaminant methyl-tert-butyl ether (hereafter also referred to as xe2x80x9cMTBExe2x80x9d). It should be understood that the present invention has applicability to the treatment of other xe2x80x9cetherxe2x80x9d contaminants, as will be described below.
MTBE has been used in xe2x80x9cpremiumxe2x80x9d gasoline since 1979 as a high octane additive which functions as an oxygenate. Its use has replaced lead and other additives such as benzene, toluene, ethylbenzene and xylenes, which are often referred to as xe2x80x9cBTEXxe2x80x9d and which are considered highly contaminating materials. More recently, for areas of the country with relatively high air pollution, the 1990 Clean Air Act requires that oxygenates be used in all grades of gasoline to reduce vehicle emissions which constitute air toxics, for example, carbon monoxide and volatile organic compounds (VOCs). Oxygenates cause fuel to burn more cleanly, reducing the amounts of ozone, carbon monoxide, toxics and other pollutants present in vehicle emissions. The current goal of gasoline reformulation is to reduce gasoline""s benzene content by 33% and other contaminating organics by at least 15%.
MTBE is the most widely used oxygenate in the United States. In 1992, more than 1.8 billion gallons of MTBE was used in gasoline. Its use has continued to increase each year since 1992 (Anderson, xe2x80x9cHealth Studies Indicate MTBE is a Safe Gasoline Additive,xe2x80x9d Chemical and Engineering News, Sep. 20, 1993). MTBE producers have invested billions of dollars into plants already in operation or planned. More than 29 companies now produce MTBE in the U.S. And in 1993, production of MTBE exceeded 24 billion gallons, making it second on the list of organic chemicals produced in the U.S. (M. S. Reisch, Chemical and Engineering News, Apr. 11, 1994; p. 12-15).
The toxicity of MTBE is still in question. An Italian study suggested that MTBE poses a significant cancer risk (Trenton Times, Nov. 13, 1994). Other studies have suggested that MTBE is not very toxic to humans (Anderson, xe2x80x9cHealth Studies Indicate MTBE is Safe Gasoline Additive,xe2x80x9d Chemical and Engineering News, 9-18, Sep. 20, 1993).
Without regard to whether MTBE is or is not toxic, it is a fact that as an ether, it has relatively low odor and taste thresholds compared to other organic compounds. MTBE""s odor threshold in water is about 45 to about 95 ppb. Its taste threshold in water is about 134 ppb (American Petroleum Institute 1993). This means that MTBE can be detected in drinking water through odor and taste at relatively low concentrations. The US Environmental Protection Agency has recommended an interim drinking water advisory for MTBE of 35 to 40 ppb. Based on rat model studies, the no-observable-adverse-effect-level (NOAEL) is 100 mg/kg/day (Robinson, M., R. H. Bruner, and G. R. Olson, xe2x80x9cFourteen and ninety day oral toxicity studies of methyl tertiary butyl ether in Sprague-Dawley rats,xe2x80x9d J. Am. Coll. Toxicol., 9:525-540 (1990)).
The full extent of MTBE contamination in US groundwaters has only recently been under careful assessment. A study performed as part of the US Geological Survey""s National Water-Quality Assessment Program revealed that MTBE is the second most commonly detected contaminant in urban groundwaters (Squillace P. J., J. S. Zogorski, W. G. Wilber, and C. V. Price. xe2x80x9cPreliminary assessment of the occurrence and possible sources of MTBE in groundwater in the United States, 1993-1994xe2x80x9d. Environ. Sci. Technol. 30:1721-1730 (1996)). As an example of this widespread problem, Buscheck et al. (Buscheck, T. E., D. J. Gallagher, T. R. Peargin, D. L. Kuehne, and C. R. Zuspan xe2x80x9cOccurrence and behavior of MTBE in groundwater. Proc. Nat. Groundwater Assoc. SW Focused Groundwater Conf. pp. 2-3. Anaheim, Calif. Jun. 3-4 (1998)) reviewed groundwater data from 700 service station sites in the US and observed that  greater than 80% of the active sites and 74% of the inactive sites had MTBE contamination. Approximately 96%, 98%, and 86% of the service station sites in Texas, Maryland, and California, respectively, that analyzed groundwater for MTBE had significant MTBE contamination. Of these sites, 63%, 82%, and 47%, respectively, had MTBE concentrations greater than 1 mg/L. This widespread contamination has led to increased public and regulatory scrutiny, and a need to identify remediation technologies.
The greatest human exposure routes of MTBE are through drinking contaminated water, use of the water in cooking, and inhalation during bathing. The chances of such exposure are not insignificant since vast amounts of MTBE-containing gasoline are stored in underground storage tanks, including tanks that leak. Seepage of MTBE from leaky tanks into groundwater and spillage of MTBE during tank filling operations and transfer operations at distribution terminals have led to considerable contamination of groundwater near these tanks. Because MTBE is highly soluble in water (43,000 ppm), it is now often found as plumes in groundwater near service stations, related storage facilities and filling terminals throughout the United States (American Petroleum Institute, Chemical Fate and Impact of Oxygenates in Groundwater: Solubility of BTEX from Gasoline-Oxygenate Mixtures,xe2x80x9d Pub. No. 4531, 1991). A market survey by The Jennings Group (1993) estimated that there are greater than 234,000 federally regulated contaminated underground storage tank (UST) sites in the United States and greater than 42,000 hazardous sites.
The recalcitrance of MTBE relative to other gasoline components makes it particularly resistant to inexpensive biological treatment approaches such as bioventing or biosparging. Conversion or xe2x80x9cremediationxe2x80x9d of the contaminated media to innocuous, environmentally-acceptable compounds, therefore, has been particularly difficult. Furthermore, MTBE can be difficult to air strip from ground water and trap on activated carbon, thereby limiting air sparging/soil vapor extraction (AS/SVE) approaches to remediation. In a recent study of 15 sites, stripping efficiencies of as low as 56% were observed (American Petroleum Institute, supra). And yet this method has been deemed to be the most effective available method for remediating contaminated groundwater.
There are other ether-based compounds that are also widely used and that are considered contaminants. Examples of such ether-based compounds include cycloaliphatic compounds, for example, tetrahydrofuran, a widely used solvent. Examples of other aliphatic ethers which are considered contaminants are ethyl-tert-butyl ether (xe2x80x9cETBExe2x80x9d), tert-amyl methyl ether (xe2x80x9cTAMExe2x80x9d) and diisopropyl ether (xe2x80x9cDIPExe2x80x9d), which are used as gasoline oxygenates.
As production of such ether-based compounds continues to grow, it can be expected that the incidence and severity of spills will increase and that the threat to the water supply will become more severe. The present invention is related to the biological treatment of ether compounds to counter such a threat by providing means to efficiently remediate contaminated sites.
Relatively little work has been done to develop means for biodegrading the aforementioned ethers. In one study, an aerobic consortia isolated from acclimated sludge was maintained on MTBE which served as the sole source of carbon for the consortia (Salanitro, J. P., L. A. Diaz, M. P. Williams, and H. L. Wimiewski, xe2x80x9cIsolation of a Bacterial Culture that Degrades Methyl t-Butyl Ether,xe2x80x9d Applied and Environmental Microbiology, Jul. 1994). MTBE was degraded to tertiary-butyl alcohol (xe2x80x9cTBAxe2x80x9d) which was also degraded by the enrichment culture. The consortium is described as comprising at least 6 different uncharacterized bacteria. The physiology of the individual organisms is not reported. It is reported that the consortia appear to have a significant population of nitrifying bacteria.
This culture has been the focus of other remediation demonstrations, including its direct injection into an MTBE-contaminated aquifer at Port Huemeome, Calif. (Salinitro, J. P., C.-S. Chou, H. L Wisniewski, and T. E. Vipond, xe2x80x9cPerspectives on MTBE biodegradation and the potential for in situ aquifer bioremediationxe2x80x9d Proceedings of the National Groundwater Association South West Focused Groundwater Conference pp. 40-54. Anaheim, Calif. Jun. 3-4 (1998)). Other studies have shown that MTBE degraders are present in sewage sludge (Cowan, R. M. and K. Park, xe2x80x9cBiodegradation of gasoline oxygenates MTBE, ETBE, TAME, TBA, and TAA by aerobic mixed culturesxe2x80x9d, Proc. 28th Mid-Atlantic Industrial and Hazardous Waste Conference (1996); Park, K. and R. Cowan, xe2x80x9cEffects of oxygen and temperature on the biodegradation of MTBExe2x80x9d, American Chemical Society National Meeting, 37:421-423 (1997)), soils (Yeh, C. K. and J. T. Novak. xe2x80x9cAnaerobic biodegradation of gasoline oxygenates in soilxe2x80x9d, Water Environment Research, 66:744-752. (1994)), river sediments (Bradley, P. M., J. E. Landmeyer, and F. H. Chapelle. xe2x80x9cAerobic mineralization of MTBE and tert-butyl alcohol by stream-bed sediment microorganismsxe2x80x9d, Environ. Sci. Technol., 33:1877-1879 (1999)), and a biofilter inoculated with groundwater (Fortin, N. Y. and M. A. Deshusses, xe2x80x9cTreatment of methyl tert-butyl ether vapors in biotrickling filters. 2. Analysis of the rate-limiting step and behavior under transient conditionsxe2x80x9d, Environmental Science and Technology, 33:2987-2991 (1999)). At least partial MTBE degradation has been observed in pure cultures of bacteria (Mo, K., C. O. Lora, A. E. Wanken, M. Javanmardian, X., Yang, and C. F. Kulpa, xe2x80x9cBiodegradation of methyl tert-butyl ether by pure bacterial culturesxe2x80x9d, Applied and Environmental Microbiology, 47:69-72 (1997); Steffan, R. J., K. McClay, S. Vainberg, C. W. Condee, and D. Zhang. Biodegradation of the gasoline oxygenates methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) by propane oxidizing bacteriaxe2x80x9d, Applied and Environmental Microbiology, 63:4216-4222, (1997)) and fungi (Hardison, L. K., S. S. Curry, L. M. Ciuffetti, and M. R. Hyman, xe2x80x9cMetabolism of diethyl ether and cometabolism of methyl tert-butyl ether by a filamentous fungus, a Graphium sp.xe2x80x9d, Applied and Environmental Microbiology 63:3059-3067 (1997)), and one recent study demonstrated growth of a pure culture of Sphingomonas strain PM1 on MTBE as a sole carbon source (Hanson, J. R., C. E. Ackerman, and K. M. Skow, xe2x80x9cBiodegradation of methyl tert-butyl ether by a bacterial pure culturexe2x80x9d Applied and Environmental Microbiology, 65:4788-4792(1999)).
It appears that in situ degradation of MTBE in aquifers also has not been studied extensively. However, recent unpublished studies by researchers at Mobile Oil Corporation have provided evidence, based on historical concentrations of MTBE in groundwater, that natural attenuation of MTBE may occur over very long periods of time in aquifers. Apparent degradation occurs after the concentrations of benzene, toluene, ethylbenzene or xylene (BTEX) are reduced to low levels. The identity of the organisms responsible for the decline in MTBE was not reported. In further studies, it was observed that MTBE was partially transformed in only one of several anaerobic sediment samples tested (Mormile et al. xe2x80x9cAnaerobic Biodegradation of Gasoline Oxygenates: Extrapolation of Information to Multiple Sites and Redox Conditions,xe2x80x9d Environ. Sci. Technol., 28:1727-1732, 1994). Transformation of MTBE in the one active sample required more than 152 days of incubation, resulted in only about 50% transformation of MTBE, and produced nearly stoichiometric amounts of TBA as a terminal product. It was reported also that MTBE was not degraded by resting cells of two anaerobic bacteria, Acetobacterium woodii and Eubacterium limosum, which, however, were effective in degrading several un-branched ethers. The authors of the study concluded that MTBE was recalcitrant to both aerobic and anaerobic biodegradation.
In view of the state of the art, it is clear that there is a need for technology that will provide the means for a rapid, efficient and cost effective process for converting MTBE and other environmentally undesirable ether-based compounds into environmentally acceptable compounds.
In accordance with the present invention, there is provided a method for degrading an ether comprising contacting said ether with a hydrogen-oxidizing bacteria. The present invention also provides a method for degrading a tertiary alcohol comprising contacting the tertiary alcohol with a hydrogen-oxidizing bacteria.
Examples of preferred species of hydrogen-oxidizing bacteria are members of the genus Hydrogenphaga, in particular Hydrogenphaga flava strain ENV735. The present invention also provides for isolating hydrogen-oxidizing bacteria by enrichment culture with hydrogen or MTBE as an electron donor.
Another aspect of the present invention includes degrading the ether by contacting it with the bacteria in a bioreactor, for example, a membrane bioreactor or a fluid bed bioreactor.
Still another aspect of the present invention includes degrading the ether with the bacteria in situ. In embodiments utilizing in situ degradation, hydrogen may be added to the subsurface or hydrogen may be produced in situ by passing an electrical current between electrodes.
The present invention provides means for degrading an ether, for example, tert-butyl ethers and/or tert-butyl alcohols efficiently and economically. It can be used to completely degrade these compounds to innocuous compounds, such as CO2 and water. In contrast, the use of prior art techniques result in undesirable degradation products which require further treatment, such as air- or steam-stripping, use of adsorbents such as activated carbon, and large expenditures of energy to burn the contaminant and the associated media. Other advantages of the present invention will become apparent from a consideration of the following detailed description of the invention.